Optical lithography is one of the key processes driving the semiconductor industry. For example, in dynamic random access memories (DRAM), decreasing the minimum lithographic dimension (the smallest image that can be printed) has accounted for about 60% of the reduction in silicon area used by DRAM chips. For logic chips, channel length is one of the key components of performance (devices having shorter channels lengths are faster) and is driven to a significant extent by optical lithography and minimum lithographic dimensions.
However, as image sizes have shrunk, it has been increasing difficult to print very small dimensions even with advanced lithographic tool and processes. To counter this difficulty, trim technology has been developed. In normal trim technology, the original image which is at the lithographic limit is “trimmed” to a sub-lithographic dimension but the dimensional tolerance on the trimmed image is not “trimmed” but is the same as on the original image, so tolerance does not scale with image size. Shadow trim allows sub-lithographic images to be formed and is illustrated in FIGS. 1 and 2 and described below. However, shadow trim technology suffers the problem that the edges of trimmed images are not “sharp” but “fuzzy.” These two problems with normal and shadow trim technology lead to formation of devices that have a wide range of performance parameters that often offset the gains obtained by making the individual devices smaller.
FIG. 1 is partial cross-sectional view of a related art shadow lithographic process. FIGS. 1 and 2 illustrate shadow trim technology as applied to ion implantation. In FIG. 1, formed on a substrate 100 are shadow masks 105. Each shadow mask 105 has a shadow sidewall 110 and an exposed sidewall 115. Each shadow mask 105 is “H1” high and are spaced “W1” apart. An angled ion implantation (in the present example, BF2+ ions at an angle φ is performed. Shadow sidewall 110 projects a shadow region 120 extending from shadow sidewall 110 into substrate 100 between shadow masks 105. Shadow region 120 is “S1” wide. “S1” is equal to “H1” ×tangent φ. Shadow region 120 includes an un-implanted region 125 and a transition region 130. Extending from exposed sidewall 115 to shadow region 120 is a fully implanted region 135. Fully implanted region 135 is “D1” wide. “D1” is equal to “W1”−“S1.” Transition region 130 is located between un-implanted region 125 and fully implanted region 135. The interface between un-implanted region 125 and transition region 130 is designated by the letter “B.” The interface between transition region 130 and fully implanted region 135 is designated by the letter “A.”
FIG. 2 is a plot of the relative amount of BF2+ implanted as a function of horizontal distance for the related art process illustrated in FIG. 1. In FIG. 2, it is seen that the width of transition region, which is the tolerance on “D1” (defined between points “A” and “B” in FIG. 1) is about 60 Å. Addition of 60 Å to the tolerance on “D1” (see FIG. 1) is unacceptable in technologies where the total tolerance allowed for all causes is 300 Å or less.
Clearly, what is needed is a method of forming sub-lithographic images having small transition regions and dimensional tolerances that scale with the image dimensions.